1. Field of the Invention
The present invention relates to a gas turbine that obtains rotational power by combusting a mixture of fuel and compressed air and supplying generated combustion gas to a turbine.
2. Description of the Related Art
Generally, a gas turbine includes a compressor, a combustor, and a turbine. Air taken in from an air-intake port is compressed by the compressor to form compressed air with high temperature and high pressure. Fuel is supplied to the compressed air to be combusted in the combustor. The turbine is driven by combustion gas with high temperature and high pressure, and a generator connected to the turbine is driven. The turbine includes a plurality of stator vanes and rotor blades arranged alternately in a casing. The turbine rotationally drives an output shaft coupled with the generator by driving the rotor blades utilizing the combustion gas. The combustion gas driven the turbine is converted to static pressure by a diffuser in an exhaust casing, and is discharged to the atmosphere.
In this type of turbine, a turbine shaft to which the rotor blades are fixed is rotatably supported in the casing via bearing portions at inlet casing side and an exhaust casing side, respectively. Lubricant is supplied to each bearing portion and a multistage seal ring is provided near the bearing portion such that the supplied lubricant does not flow out of the bearing portion. By supplying air extracted from the compressor to the seal ring, leakage of the lubricant from the bearing portion is suppressed.
Such a gas turbine is described in Japanese Patent Application Laid-Open No. 2005-023812.
However, at the exhaust casing side in the gas turbine, an exhaust diffuser that converts the exhaust gas to static pressure to discharge the gas to the outside is provided on an outer peripheral side of the exhaust-side bearing portion, and the bearing portion is heated by exhaust gas with high temperature flowing in the exhaust diffuser. Particularly, when the gas turbine is operated with a low output for a long time, high temperature gas accumulates inside the exhaust diffuser to raise the temperature of the exhaust-side bearing portion, and lubricant supplied to the exhaust-side bearing portion is carbonized to be solidified. As a result, carbide of lubricant adheres on a seal face of the seal ring and the carbide contacts with the turbine shaft, which can generate vibrations.